Semiconductor device

ABSTRACT

A planar MOSFET is provided on the upper surface of the N − -type semiconductor substrate in a mesa portion between the trenches. A P + -type emitter layer is provided between the trench and the planar MOSFET in the mesa portion. A P-type collector layer is provided on a lower surface of the N − -type semiconductor substrate. The planar MOSFET includes an N + -type emitter layer, an upper portion of the N − -type semiconductor substrate, a P-type base layer, and a planar gate on the foregoing with a gate insulating film interposed therebetween. The planar gate is connected to the gate trench. The P + -type emitter layer has a higher impurity concentration than the P-type base layer and has an electric potential equal to an emitter potential of the N + -type emitter layer. The N + -type emitter layer is not in contact with the trench. A trench MOSFET is not formed.

FIELD

The present invention relates to a semiconductor device including anInsulated Gate Bipolar Transistor (IGBT).

BACKGROUND

In planar IGBTs having planar MOS structures, a planar gate is used.Accordingly, a region necessary for device operation must be ensured,and miniaturization is limited. Moreover, a high ON voltage imposes alimitation. Meanwhile, trench IGBTs have trench (vertical) gatestructures, and can therefore be miniaturized (e.g., see Patent Document1). Further, ON voltage characteristics can be improved by utilizing theelectron injection effect at trench bottoms.

CITATION LIST Patent Literature

-   Patent Document 1: Japanese Patent Application Publication No.    2000-228519

SUMMARY Technical Problem

However, trench IGBTs have the problem that short-circuit breakingcapacities are low because saturation current densities are high due tohigh active cell densities. A cell is a minimum pattern repeated inperpendicular and longitudinal directions of trenches. An active celldensity is the number of cells within an area of 1 cm². An active celldensity is defined by the following equation:

active cell density=1/(minimum repeated-unit size in the perpendiculardirection of trenches*minimum repeated-unit size in the longitudinaldirection of trenches)

Moreover, in the case where the number (hereinafter referred to as athinning ratio) of trenches per cell cannot be increased, a saturationcurrent can be reduced by increasing a width of a P⁺-type emitter layerin the longitudinal direction of the trenches, but the ON voltageincreases. A thinning ratio is the ratio of the number of trenches setto the emitter potential to the total number of trenches within a cell.A thinning ratio is defined by the following equation:

thinning ratio=the number of trenches set to the emitter potentialwithin a cell/the total number of trenches within the cell

The present invention has been accomplished to solve the above-describedproblems, and an object of the present invention is to provide asemiconductor device in which a saturation current can be reducedwithout adverse effects on an ON voltage.

Solution to Problem

A semiconductor device according to the present invention includes: anN-type semiconductor substrate; a plurality of trenches in an uppersurface of the N-type semiconductor substrate; a gate trench in thetrench with an insulating film interposed therebetween; a planar MOSFETon the upper surface of the N-type semiconductor substrate in a mesaportion between the trenches; a P-type emitter layer between the trenchand the planar MOSFET in the mesa portion; and a P-type collector layeron a lower surface of the N-type semiconductor substrate, wherein theplanar MOSFET includes an N-type emitter layer, an N-type diffusionlayer connected to the N-type semiconductor substrate, a P-type baselayer between the N-type emitter layer and the N-type diffusion layer,and a planar gate on a part of the N- type emitter layer, the N-typediffusion layer and the P-type base layer with a gate insulating filminterposed therebetween, the planar gate is connected to the gatetrench, the P-type emitter layer has a higher impurity concentrationthan the P-type base layer and has an electric potential equal to anemitter potential of the N-type emitter layer, the N-type emitter layeris not in contact with the trench, and a trench MOSFET is not formed.

Advantageous Effects of Invention

In the present invention, the P-type emitter layer having a highimpurity concentration is provided between the trench and the planarMOSFET, and the N-type emitter layer is not in contact with the trench.Accordingly, there is no path in which an electronic current flows alongsides of the trenches. Therefore, a resistance component of the pathdoes not exist, and therefore an ON voltage is not adversely affected.Further, by lowering the active cell density by increasing the channellength of the planar MOSFET without increasing the length of the P-typeemitter layer, a saturation current can be reduced without adverseeffects on the ON voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective cross-sectional view showing a semiconductordevice according to Embodiment 1 of the present invention.

FIG. 2 is a perspective cross-sectional view in which a planar gate ofthe device in FIG. 1 is omitted.

FIG. 3 is a perspective cross-sectional view in which the planar gateand a gate insulating film of the device in FIG. 1 are omitted.

FIG. 4 is a cross-sectional view taken along I-II of FIG. 3.

FIG. 5 is a plan view showing the planar MOSFET according to Embodiment1 of the present invention.

FIG. 6 is a cross-sectional view showing a planar IGBT according toComparative Example 1.

FIG. 7 is a perspective cross-sectional view showing a trench IGBTaccording to Comparative Example 2.

FIG. 8 is a plan view for explaining the flow of an electronic currentin Comparative Example 2.

FIG. 9 is a plan view for explaining the flow of an electronic currentin the semiconductor device according to Embodiment 1 of the presentinvention.

FIG. 10 is a view showing the channel width dependence of the saturationcurrent density Jc(sat).

FIG. 11 is a view showing the channel length dependence of thesaturation current density Jc(sat).

FIG. 12 is a plan view showing a modified example of the planar MOSFETaccording to Embodiment 1 of the present invention.

FIG. 13 is a perspective cross-sectional view showing a modified exampleof the semiconductor device according to Embodiment 1 of the presentinvention.

FIG. 14 is a perspective cross-sectional view showing a semiconductordevice according to Embodiment 2 of the present invention.

FIG. 15 is a cross-sectional view taken along I-II of FIG. 14.

FIG. 16 is a cross-sectional view showing a semiconductor deviceaccording to Embodiment 3 of the present invention.

FIG. 17 is a view showing ON voltages of the semiconductor devicesaccording to Comparative Example 1 and Embodiments 1 to 3.

FIG. 18 is a perspective cross-sectional view showing a semiconductordevice according to Embodiment 4 of the present invention.

FIG. 19 is a perspective cross-sectional view in which a planar gate ofthe device in FIG. 18 is omitted.

FIG. 20 is a perspective cross-sectional view in which the planar gateand a gate insulating film of the device in FIG. 18 are omitted.

FIG. 21 is a cross-sectional view taken along I-II of FIG. 20.

FIG. 22 is a view showing J_(c)-V_(c) output characteristic waveforms ofthe devices of Comparative Examples 1 and 2 and Embodiments 1 and 4which have the same channel length.

FIG. 23 is a view showing short-circuit breaking capacities Jc of thedevices of Comparative Examples 1 and 2 and Embodiments 1 and 4.

FIG. 24 is a perspective cross-sectional view showing a modified exampleof the semiconductor device according to Embodiment 4 of the presentinvention.

FIG. 25 is a perspective cross-sectional view showing a semiconductordevice according to Embodiment 5 of the present invention.

DESCRIPTION OF EMBODIMENTS

A semiconductor device according to the embodiments of the presentinvention will be described with reference to the drawings. The samecomponents will be denoted by the same symbols, and the repeateddescription thereof may be omitted.

Embodiment 1

FIG. 1 is a perspective cross-sectional view showing a semiconductordevice according to Embodiment 1 of the present invention. FIG. 2 is aperspective cross-sectional view in which a planar gate of the device inFIG. 1 is omitted. FIG. 3 is a perspective cross-sectional view in whichthe planar gate and a gate insulating film of the device in FIG. 1 areomitted. FIG. 4 is a cross-sectional view taken along I-II of FIG. 3. Itshould be noted that though an example of a high breakdown voltage classof 6500 V is described as an embodiment, the present invention can beapplied to any breakdown voltage class.

A plurality of trenches 2 are provided in an upper surface of an N⁻-typesemiconductor substrate 1. A gate trench 4 is provided in the trench 2with an insulating film 3 interposed therebetween. There is a planarMOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) 5 on theupper surface of the N⁻-type semiconductor substrate 1 in a mesa portionbetween the trenches 2. A P⁺-type emitter layer 6 is provided betweenthe trench 2 and the planar MOSFET 5 in the mesa portion. An N-typebuffer layer 7 and a P-type collector layer 8 are provided in order on alower surface of the N⁻-type semiconductor substrate 1. A collectorelectrode 9 is connected to the P-type collector layer 8.

The planar MOSFET 5 includes an N⁺-type emitter layer 10, an upperportion of the N⁻-type semiconductor substrate 1, a P-type base layer 12provided between the N⁺-type emitter layer 10 and the upper portion ofthe N⁻-type semiconductor substrate I, and a planar gate 14 provided onthe foregoing with a gate insulating film 13 interposed therebetween.The N⁺-type emitter layer 10 serves as a source, the upper portion ofthe N⁻-type semiconductor substrate 1 serves as a drain, and the P-typebase layer 12 serves as a channel. Thus, the planar MOSFET 5 operates asan n-channel MOSFET. The gate trench 4 and the planar gate 14 arepolysilicon, and the insulating film 3 and the gate insulating film 13are oxide films.

The planar gate 14 is connected to the gate trench 4. The P⁺-typeemitter layer 6 is provided between the trench 2 and the N⁺-type emitterlayer 10. The P⁺-type emitter layer 6 has a higher impurityconcentration than the P-type base layer 12, and has an electricpotential equal to an emitter potential of the N⁺-type emitter layer 10.The N⁺-type emitter layer 10 is not in contact with the trench 2, and atrench MOSFET is not formed.

FIG. 5 is a plan view showing the planar MOSFET according to Embodiment1 of the present invention. It should be noted that the gate insulatingfilm 13 and the planar gate 14 are omitted. The N⁺-type emitter layer10, the P-type base layer 12, and the upper portion of the N⁻-typesemiconductor substrate 1 are arranged in order in the longitudinaldirection of the trenches 2 in planar view perpendicular to the uppersurface of the N⁻-type semiconductor substrate 1.

A width of the P-type base layer 12 in the lateral direction of thetrenches 2 in planar view is a channel width W. A length of the P-typebase layer 12 in the longitudinal direction of the trenches 2 in planarview is a channel length L. The active cell density in the longitudinaldirection of the trenches 2 can be adjusted by adjusting the length L.

Next, effects of the present embodiment will be described whilecomparisons with Comparative Examples 1 and 2 are made. FIG. 6 is across-sectional view showing a planar IGBT according to ComparativeExample 1. FIG. 7 is a perspective cross-sectional view showing a trenchIGBT according to Comparative Example 2. In Comparative Example 2, aplurality of dummy trenches 15 are provided in the upper surface of theN⁻-type semiconductor substrate 1. A dummy gate trench 17 is provided inthe dummy trench 15 with an insulating film 16 interposed therebetween.The dummy gate trench 17 has an electric potential equal to the emitterpotential of the N⁺-type emitter layer 10. Moreover, an N-type diffusionregion 18 is provided between the N⁻-type semiconductor substrate 1 andthe P-type base layer 12.

In a withstanding mode (V_(ge)=0 V, V_(ce)=V_(cc)), the gate trenches 4and the dummy trenches 15 act as field plates. Accordingly, ComparativeExample 2 achieves a higher breakdown voltage than that of ComparativeExample 1 under conditions where the thickness and resistivity of theN⁻-type semiconductor substrate 1 are the same.

FIG. 8 is a plan view for explaining the flow of an electronic currentin Comparative Example 2. FIG. 9 is a plan view for explaining the flowof an electronic current in the semiconductor device according toEmbodiment 1 of the present invention. Paths of electronic currents areindicated by arrows.

In the case of Comparative Example 2, the length of the P⁺-type emitterlayer 6 is increased to lower the active cell density and increase aresistance component, thus reducing a saturation current densityJc(sat). However, a path in which an electronic current flows alongsides of the trenches 2 has a resistance component, and an ON voltage isadversely affected.

Meanwhile, in the present embodiment, the P⁺-type emitter layer 6 havinga high impurity concentration is provided between the trench 2 and theplanar MOSFET 5, and the N⁺-type emitter layer 10 is not in contact withthe trench 2. Accordingly, there is no path in which an electroniccurrent flows along sides of the trenches 2. Accordingly, an electroniccurrent flows directly under a channel of the planar MOSFET 5 and theP⁺-type emitter layer 6. As a result, a resistance component such as ina trench IGBT does not exist, and therefore an ON voltage is notadversely affected. Further, by lowering the active cell density byincreasing the channel length L of the planar MOSFET 5 withoutincreasing the length of the P⁺-type emitter layer 6, a saturationcurrent can be reduced without adverse effects on the ON voltage.

FIG. 10 is a view showing the channel width dependence of the saturationcurrent density Jc(sat). FIG. 11 is a view showing the channel lengthdependence of the saturation current density Jc(sat). Evaluationconditions are V_(GE)=13.5 V, V_(CE)=20 V, and T_(j)=25° C. In FIG. 10,the channel length is 4 μm. In FIG. 11, the channel width is 2 μm. Inthe case where the gate voltage is constant, Jc(sat) is a characteristicwhich indicates the current driving force of the device per unit area.From FIGS. 10 and 11, it can be seen that Jc(sat) drastically decreaseswhen the channel width is smaller than 0.3 μm and that Jc(sat)drastically decreases when the channel spacing is smaller than 3.0 μm.Accordingly, it is preferable that the channel width W is not less than0.3 μm and that the channel length L is not less than 3.0 μm.

FIG. 12 is a plan view showing a modified example of the planar MOSFETaccording to Embodiment 1 of the present invention. The planar gate 14of the planar MOSFET 5 is divided. In this case, again, the same effectsas those of the above-described embodiment can be obtained.

FIG. 13 is a perspective cross-sectional view showing a modified exampleof the semiconductor device according to Embodiment 1 of the presentinvention. There is no N-type buffer layer 7 on the lower surface of theN⁻-type semiconductor substrate 1. In this case, again, the same effectsas those of the above-described embodiment can be obtained.

Moreover, the trench 2 shown in the above-described embodiment has around-bottomed shape. It should be noted, however, that the presentinvention is not limited to this. For example, even when the trenches 2having other shape such as a rectangular bottom or an expanded bottom isused, the same effects as those of the above-described embodiment can beobtained.

Embodiment 2

FIG. 14 is a perspective cross-sectional view showing a semiconductordevice according to Embodiment 2 of the present invention. FIG. 15 is across-sectional view taken along I-II of FIG. 14. An N-type diffusionlayer 19 serving as the drain of the planar MOSFET 5 is providedthroughout the entire cell region. The N-type diffusion layer 19 isconnected to the N⁻-type semiconductor substrate 1, has a higherimpurity concentration than the N⁻-type semiconductor substrate 1, andis at a shallower depth than the trenches 2. The N-type diffusion layer19 serves as a barrier layer against holes, and the carrierconcentration on the emitter side of the device increases. Accordingly,the ON voltage can be reduced. Other components and effects are the sameas those of Embodiment 1.

Embodiment 3

FIG. 16 is a cross-sectional view showing a semiconductor deviceaccording to Embodiment 3 of the present invention. The N-type diffusionlayer 19 is provided under part of the planar gate 14. Other componentsare the same as those of Embodiment 2. In this case, again, the sameeffects as those of Embodiment 2 can be obtained.

FIG. 17 is a view showing ON voltages of the semiconductor devicesaccording to Comparative Example 1 and Embodiments 1 to 3. Evaluationconditions are V_(GE)=15 V, J_(c)=rated current density, and T_(j)=25°C. It can be seen that in Embodiments 2 and 3, the ON voltage furtherdecreases compared to that of Embodiment 1.

Embodiment 4

FIG. 18 is a perspective cross-sectional view showing a semiconductordevice according to Embodiment 4 of the present invention. FIG. 19 is aperspective cross-sectional view in which a planar gate of the device inFIG. 18 is omitted. FIG. 20 is a perspective cross-sectional view inwhich the planar gate and a gate insulating film of the device in FIG.18 are omitted. FIG. 21 is a cross-sectional view taken along I-II ofFIG. 20.

A plurality of dummy trenches 15 are provided in an upper surface of theN⁻-type semiconductor substrate 1. A dummy gate trench 17 is provided inthe dummy trench 15 with an insulating film 16 interposed therebetween.The dummy gate trench 17 has an electric potential equal to an emitterpotential of the N⁺-type emitter layer 10. The dummy gate trench 17 ispolysilicon, and the insulating film 16 is an oxide film.

There is a planar MOSFET 5 on the upper surface of the N⁻-typesemiconductor substrate 1 in a mesa portion between the trenches 2, butthere is no planar MOSFET 5 between the dummy trenches. This increasesthe thinning ratio in the lateral direction of the trenches 2 in planarview and decreases the active cell density. Thus, Jc(sat) can bereduced. Other components and effects are the same as those ofEmbodiment 1.

FIG. 22 is a view showing J_(c)-V_(c) output characteristic waveforms ofthe devices of Comparative Examples 1 and 2 and Embodiments 1 and 4which have the same channel length. Evaluation conditions areV_(GE)=13.5 V and T_(j)=25° C. The thinning ratio of Embodiment 4 wasset to the same value as a small thinning ratio of Comparative Example2. It can be seen that in Embodiments 1 and 4, the saturation current Jccan be reduced without adverse effects on the ON voltage Vc.

FIG. 23 is a view showing short-circuit breaking capacities Jc of thedevices of Comparative Examples 1 and 2 and Embodiments 1 and 4.Evaluation conditions are V_(CC)=4500 V, V_(GE)=15 V, and T_(j)=125° C.An indicator of short-circuit breaking capacity is a maximum pulse widthTW capable of disconnection without breaking the device. Here, TW ofComparative Example 1 is defined as 1, and Jc(sat) of ComparativeExample 1 is defined as 1. As Jc(sat) increases, heat generated in thedevice during short-circuiting increases, and short-circuit withstandingtime (TW) decreases. It can be seen that in Embodiment 4, since Jc(sat)is reduced, a higher short-circuit breaking capacity is achieved.

FIG. 24 is a perspective cross-sectional view showing a modified exampleof the semiconductor device according to Embodiment 4 of the presentinvention. One gate trench 4 is sandwiched between two mesa portions. Inthis case, again, the same effects as those of the above-describedembodiment can be obtained.

Embodiment 5

FIG. 25 is a perspective cross-sectional view showing a semiconductordevice according to Embodiment 5 of the present invention. Across-sectional view taken along I-II of FIG. 25 is the same as that ofFIG. 15. An N-type diffusion layer 19 serving as the drain of the planarMOSFET 5 is provided throughout the entire cell region. The N-typediffusion layer 19 has a higher impurity concentration than that of theN⁻-type semiconductor substrate 1, and is at a shallower depth than thetrenches 2. The N-type diffusion layer 19 serves as a barrier layeragainst holes, and the carrier concentration on the emitter side of thedevice increases. Accordingly, the ON voltage can be reduced. Othercomponents and effects are the same as those of Embodiment 4. Moreover,in Embodiment 5, the N-type diffusion layer 19 may be provided underpart of the planar gate 14 as in Embodiment 3. In this case, again, thesame effects as those of Embodiment 5 can be obtained.

It should be noted that the semiconductor device is not limited to beingmade of silicon, and may be made of a wide band gap semiconductor havinga wider band gap than silicon. Examples of the wide band gapsemiconductor are, for example, silicon carbide, gallium nitride-basedmaterials, and diamond. A semiconductor device made of such a wide bandgap semiconductor has a high breakdown voltage and a high allowablecurrent density, and can therefore be miniaturized. By using theminiaturized device, a semiconductor module into which the device isincorporated can also be miniaturized. Further, since the heatresistance of the device is high, radiation fins of a heat sink can beminiaturized, and a water-cooled portion can be changed to an air-cooledportion. Accordingly, the semiconductor module can be furtherminiaturized. Moreover, by virtue of small power loss in the device andhigh efficiency thereof, the efficiency of the semiconductor module canbe improved.

REFERENCE SIGNS LIST

-   1 N⁻-type semiconductor substrate; 2 trench; 3,16 insulating film; 4    gate trench; 5 planar MOSFET; 6 P⁺-type emitter layer; 8 P-type    collector layer; 10 N⁺-type emitter layer; 12 P-type base layer; 13    gate insulating film; 14 planar gate; 15 dummy trench; 17 dummy gate    trench; 19 N-type diffusion layer

1. A semiconductor device comprising: an N-type semiconductor substrate;a plurality of trenches in an upper surface of the N-type semiconductorsubstrate; a gate trench in the trench with an insulating filminterposed therebetween; a planar MOSFET on the upper surface of theN-type semiconductor substrate in a mesa portion between the trenches; aP-type emitter layer between the trench and the planar MOSFET in alateral direction of the trench in planar view perpendicular to theupper surface of the N-type semiconductor substrate in the mesa portion;and a P-type collector layer on a lower surface of the N-typesemiconductor substrate, wherein the planar MOSFET includes an N-typeemitter layer, an upper portion of the N-type semiconductor substrate, aP-type base layer between the N-type emitter layer and the upper portionof the N-type semiconductor substrate, and a planar gate on a part ofthe N-type emitter layer, the upper portion of the N-type semiconductorsubstrate and the P-type base layer with a gate insulating filminterposed therebetween, the planar gate is connected to the gatetrench, the P-type emitter layer has a higher impurity concentrationthan the P-type base layer and has an electric potential equal to anemitter potential of the N-type emitter layer, the P-type emitter layeris provided between the N-type emitter layer and the trench and betweenthe P-type base layer and the trench, the N-type emitter layer and theP-type base layer are not in contact with the insulating film in thetrench, and a trench MOSFET is not formed.
 2. The semiconductor deviceof claim 1, wherein the N-type emitter layer, the P-type base layer, andthe upper portion of the N-type semiconductor substrate are arranged inorder in a longitudinal direction of the trench in the planar view. 3.The semiconductor device of claim 2, wherein a width of the P-type baselayer in a lateral direction of the trench in the planar view is notless than 0.3 μm.
 4. The semiconductor device of claim 2, wherein adistance between the P-type base layer and an adjacent P-type base layerin a longitudinal direction of the trench in the planar view is not lessthan 3.0 μm.
 5. The semiconductor device of claim 1, further comprisingan N-type diffusion layer in the upper portion of the N-typesemiconductor substrate and having a higher impurity concentration thanthe N-type semiconductor substrate, and having a shallower depth thanthe trench.
 6. The semiconductor device of claim 1, further comprising:a plurality of dummy trenches in the upper surface of the N-typesemiconductor substrate; and a dummy gate trench in the dummy trenchwith an insulating film interposed therebetween and having an electricpotential equal to the emitter potential of the N-type emitter layer.